The present invention relates to a sensed-pressure-data converter having a circuit for reducing an output offset and an offset drift of a pressure sensitive resistance element.
Today, pressure sensitive resistance elements are widely used as various pressure sensors and keyboard switches. FIG. 14 is an explanatory diagram showing a conventional pressure sensitive resistance element. In FIG. 14, reference numeral 51 denotes an upper film as an insulating substrate, reference numeral 52 denotes a lower film also as an insulating substrate, reference numeral 53 denotes an Ag electrode pattern formed on lower film 52, reference numeral 54 denotes an Ag electrode pattern formed on upper film 51 oppositely to Ag electrode patterns 53, and reference numeral 55 denotes a pressure sensitive conductor applied and formed on the Ag electrode pattern 54. Lower film 52 and upper film 51 are disposed so that pressure sensitive conductor 55 may contact with Ag electrode pattern 53. This pressure sensitive conductor 55 is formed by mixing conductive particles in an insulating rubber material. Pressing and, compressing pressure sensitive conductor 55 makes gaps of the conductive particles in pressure sensitive conductor 55 narrower and the resistance lower. That is, as the pressure getting high, the resistance falls. In the conventional sensed-pressure-data converter, this resistance change is directly converted into a voltage.
In such conventional pressure sensitive resistance element, in general, the pressure sensitive conductor is formed by printing method in order to reduce cost, and therefore, the resistance and resistance changing characteristic vary depending on the thickness of the pressure sensitive conductor. Consequently, it was difficult to maintain the resistance at a desired value and keep fluctuations of the changing characteristic within a specific range. It was hence difficult to obtain a desired output if the resistance value of the pressure sensitive resistance element is directly converted into a voltage.
The invention is intended to solve these problems, and it is hence an object thereof to present a sensed-pressure-data converter having a circuit for reducing an output offset and an offset drift of a pressure sensitive resistance element.
To solve the problems, the sensed-pressure-data converter of the invention comprises:
a pressure sensitive resistance element having a pressure sensitive conductor interposed between two insulating substrates disposed face to face, for varying the resistance thereof corresponding to the load applied from outside within the confronting surfaces of the insulating substrates; and
controller for detecting the electric characteristic of the pressure sensitive resistance element, for converting it to a specific electric characteristic signal and for issuing it.
The controller comprises:
an A/D converter receiving a signal from the pressure sensitive resistance element;
a memory preliminarily storing a compensation value based on an error signal between the electric characteristic of the pressure sensitive resistance element and the reference electric characteristic;
a D/A converter for compensating the electric characteristic due to the resistance change of the pressure sensitive resistance element as compared with the signal from the A/D converter based on the compensation value stored in the memory, for converting it into a specific electric characteristic signal, and for issuing it; and
an adjustment value input terminal for entering the error signal into the input terminal of the A/D converter.
In this constitution, a sensed-pressure-data converter capable of reducing the output offset and the offset drift of the pressure sensitive resistance element is obtained.